I recently played the Dragon Age games again, and the way they allow you to upgrade your abilities is great. Probably one of the best. As an example, after getting Chain Lightning you can spend an additional point to allow it to hit additional targets, but this is not always a no-brainer decision. Also, you feel really rewarded after getting each new upgrade for an ability, which can sometimes even change how the ability works. Very different from a bland +10% damage boost per rank.
What I want to say here is that this kind of customization could be easily implemented on Neverwinter, simply replacing the "ranks" on powers for "upgrades". So, instead of new ranks just adding +10% damage, it could effectively change the power, adding a new effect. And I say this is easy to implement because most powers already have multiple effects, and they could just gate these effects behind upgrades.
Let me use as example the Warlock's power called Dreadtheft.
This power deals damage in area, and also debuffs the targets to take extra damage, and also increases the caster's resistance. So, instead of 4 ranks slightly boosting every aspect of the power as it does now, why not allow the player to upgrade this power as desired?
Something like: - Upgrade 1: Adds the debuff effect. - Upgrade 2: Adds the resistance buff. - Upgrade 3: Allows it to work on more targets. - Upgrade 4: Decreases cooldown.
And so on.. The player would be able to customize each power as desired.